Dependencies: Add lru-cache library

Adds a generic C++ based caching library to support material external
modules. This is a template based library which means there is nothing
to build or link.

The only modification is to the CMakeLists.txt file to enable
integration with FreeCAD,

Original source: https://github.com/goldsborough/lru-cache

src/3rdParty/lru-cache/examples/fibonacci-timed.cpp

@@ -0,0 +1,57 @@
+/// The MIT License (MIT)
+/// Copyright (c) 2016 Peter Goldsborough
+///
+/// Permission is hereby granted, free of charge, to any person obtaining a copy
+/// of this software and associated documentation files (the "Software"), to
+/// deal in the Software without restriction, including without limitation the
+/// rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
+/// sell copies of the Software, and to permit persons to whom the Software is
+/// furnished to do so, subject to the following conditions:
+///
+/// The above copyright notice and this permission notice shall be included in
+/// all copies or substantial portions of the Software.
+///
+/// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+/// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+/// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+/// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+/// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+/// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
+/// IN THE SOFTWARE.
+
+#include <chrono>
+#include <iostream>
+
+#include "lru/lru.hpp"
+
+using namespace std::chrono_literals;
+
+using Cache = LRU::TimedCache<int, int>;
+
+int fibonacci(int n, Cache& cache) {
+  if (n < 2) return 1;
+
+  // We internally keep track of the last accessed key, meaning a
+  // `contains(key)` + `lookup(key)` sequence will involve only a single hash
+  // table lookup.
+  if (cache.contains(n)) return cache[n];
+
+  auto value = fibonacci(n - 1, cache) + fibonacci(n - 2, cache);
+
+  // Caches are 100% move-aware and we have implemented
+  // `unordered_map` style emplacement and insertion.
+  cache.emplace(n, value);
+
+  return value;
+}
+
+int fibonacci(int n) {
+  // Use a time to live of 100ms. This means that 100ms after insertion, a key
+  // will be said to have "expired" and `contains(key)` will return false.
+  Cache cache(100ms);
+  return fibonacci(n, cache);
+}
+
+auto main() -> int {
+  std::cout << fibonacci(32) << std::endl;
+}